DESCRIPTION Acetylcholine (ACh) is a neurotransmitter that has powerful modulatory influences on neuronal excitability in the mammalian CNS. In contrast to well-studied excitatory actions of ACh on pyramidal cells, little is known about direct effects of cholinergic receptor activation on neocortical interneurons. To test the hypothesis that various types of interneurons in layer V of rat visual cortex respond to ACh differently due to activation of diverse type of subtypes of cholinergic receptors, whole cell current/voltage clamp recordings from visualized neurons in slices will be used in conjunction with intracellular labeling, extracellular drug application, single-cell PCR, and immunocytochemistry. Specifically, the cholinergic modulation of a membrane potential and input resistance in different types of interneurons will be examined. The involvement of different types or subtypes of ACh receptors will be assessed by (1) application of different agonists and/or antagonists; (2) single-cell PCR targeting of mRNAs coding for nicotinic alpha4 and beta2 subunits; and (3) fluorescent antibody labeling of the nicotinic subunits. The ionic mechanisms underlying the actions of ACh will be determined by analysis of voltage clamp data in conjunction with alterations of ionic concentration. The muscarinic modulation of I/A, I/M and AHP currents in different interneurons will also be investigated. This proposed study will provide new information relevant to functional modulation of cortical inhibition nd cortical information processing. Results will also have implications for neurological disorders, such as epilepsy, where cortical disinhibition is hypothesized to be an important underlying mechanism and Alzheimer's disease, in which functional abnormalities may result, at least in part, from a loss of cortical cholinergic innervation.